Nano-generator could power tiny devices

The day when you can charge your cell phone or iPod just by going for a stroll around the block could be a step closer, thanks to a "nano-generator".

About a year ago, Zhong Lin Wang of Georgia Tech in the US discovered that, when he disturbed zinc oxide nanowires, they gave off a tiny electrical current, a phenomenon called piezoelectricity. At the time, he had to use the tip of an atomic force microscope - a $250,000 instrument - to create about one-billionth of a watt of power. Not exactly energy efficiency.

But with his latest experiment, Wang has improved his design at least a thousand-fold. Using gold nanoparticle as seeds, he grew a small forest of 1-micron-high zinc oxide wires on a conductive substrate 2 millimetres square. Then he placed a saw-toothed electrode on top, which is designed to make contact with as many nanowires as possible. Finally, by rattling the whole thing with ultrasound, he found that he could generate a few microwatts of electricity.

That is still only a few millionths of a watt. But, by using ultrasound, the team demonstrated that they can activate the generator using any form of vibration. The movement of the top electrode disturbs the nanowires, providing a potential power source for anything that moves.

Furthermore, nanowires can be chemically grown on virtually any substrate, including metals, polymers, and anything else that could double as an electrode. The wires also precipitate from solution at 70°C, making them easy to grow under normal laboratory conditions.

Human implants

The generator suffers from a few key limitations, however. First, growing uniform nanowires is difficult - they are usually of slightly different height or diameter. As a result, in a generator containing many thousands of nanowires, only a few hundred or so successfully generate electricity when shaken, as they do not all make contact with the electrode. That hurdle must be overcome in order to charge large, power-hungry devices.

But Wang believes the nano-generator could be ideal for powering tiny devices, including those that may be implanted inside the human body. "Imagine self-powered force-sensors implanted in blood vessel walls, taking your blood pressure. Or generators in your shoes that can charge devices while you walk," he says.

Almost any device that could use a wireless, mobile power source could potentially use the nanogenerator, Wang says: "I have full confidence that within three years we will have something that is useful commercially."

Journal reference: Science (vol 316, p 102)

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A saw-toothed electrode is placed on top of the nanowires (Image: Science)